Reproduction

The Annual Cycle of
Reproduction

( From
the study of Dr Clifford G Rice )

The
reproductive cycle of a species is closely attuned to the ecological conditions
under which it lives. The reproductive cycle of Nilgiri tahr is
therefore of interest in light of the climatic and biotic cycles in which
it lives. This information can be compared to Caughley's (1971) work on
the closely related Himalayan tahr inhabiting a decidedly temperate regime in
New Zealand. Jerdon (1874) stated that the Nilgiri tahr "is said to
produce two young at birth", and this was apparently the prevailing
opinion when Fletcher (1911) wrote that he was more inclined to judge one to be
the usual number. Prater (1965) maintained that sometimes two, but more
commonly one young are produced. However, more recent observations in both
the wild (Davidar, 1978; Schaller, 1978; and this study) and captivity (Wilson,
'1980; Waterhouse pers. comm) indicate that one young per female is the
rule. Davidar (1978) reports one instance of twin fetuses and states that
"occasionally, a mother with two young at heel have been observed."
However, I encountered no indications of twinning during the course of my study.
So, although twins may occasionally occur, the usual number of young
is one.

Detailed information on reproduction during the study was
obtained during the birth and rutting seasons of 1981. Flighty animals,
adverse weather conditions, and the lack of individually
recognizable females precluded a comparable accounting during 1879 and
1980. The distribution of births was well charted in the winter of 1981,
and the occurrences of estrus was observed during the monsoon of that
followed. Unfortunately, the study ended before the births which resulted
from the 1981 rut. However, by good fortune, one of the first females to
be collared at the end of the 1980 rut was one of the last females to be bred,
and both the breeding and birth dates were documented. She was bred
on 03 September 1980, and gave birth on 01 March 1981, an gestation
of 179 days. This gestation period compares favorably with the
one period of 180 days reported by Winbigler (1977), and the 180
days estimated for Himalayan tahr (Caughley, 1971). This
gestation period can be used to estimate dates of conceptions leading to
the 1981 births, as a means of comparing the 1980 and 1981 rutting seasons. In
1981, the first young of the year was observed on 10 January, and this was
considered as the start of the birth season. From then until births
declined in mid-February, I attempted to find all females in the Vaguvarrai
intensive study subpopulation daily Because of the large number of collared
females, it was possible to keep track of the subsequent births on a daily
basis. There was an initial peak, with Additional births trailing off
until the last of the season on 01 April. However, there was also a
conspicuous gap in the parturitions between 23 January and 31
January.

This
distribution of births spread over a period of 80 days shows a remarkable
similarity to the skewed distribution of births calculated by Caughley (1971)
for Himalayan tahr, especially if the Nilgiri tahr distribution curve were
smoothed by probit analysis as Caughley's was. Estrous, as indicated by
copulation, tolerance of mounting, and high frequency of courtship
displays was recorded for 28 females. Only one female was recorded
in estrus twice, at an interval of 49 days (15 July-02 September).
Since nearly all sexually mature females gave birth in the main (winter) birth
season of 1981, and assuming that this was the case in 1982 (after the study
ended), the logical conclusion is that the conception rate (per estrus) was very
high (over 90%).

There were
also two distinct periods of estrous activity, separated by a gap of 32 days. In
1981 there was also a series of births which occurred during the monsoon.
Of these births, two were to collared females whose young had died early in the
season. Birw's was first seen on 12 January 1981, already dead. She gave
birth to her second young that year 197 days later. Rrw's first young was
born on 15 January 1981 and was missing 14 days later. She gave birth to
her second young 184 days after this. These records indicate that females which
lost their young early in the birth season, come into estrus again quite soon,
and conceived again. The clustering of the other monsoon births
would seem to indicate that the other females showed a similar pattern. Of
the individually recognized females which gave birth in the monsoon, two showed
definite subsequent signs of estrus 36 and 24 days later. In New Zealand,
Himalayan tahr give birth from November-January, with the mean day of
birth calculated as 26 November (Caughley, 1971). In the Himalayas, the
birth season falls from mid-April—mid- July (Schaller,
1973). These spring-early summer birth seasons are typical of
temperate ungulate species, and are presumably an
adaptation which favors survival of offspring born at the time of warming
temperatures and increasing forage production.